1. Field of the Invention
This invention relates to a motor-driven power steering system for a vehicle adapted to assist the operator-induced steering operation by means of the rotating force of a motor.
2. Description of the Prior Art
In the past, a conventional power steering system of this type, as schematically illustrated in FIG. 1, has hitherto been known in which a steering wheel 1 is operatively connected through a steering shaft 2 having an upper portion 2a and a lower portion 2b and a first rack and pinion gear T.sub.1 including a first pinion 5 and a first rack tooth portion 6a with a steering rack 6 which is connected at its opposite ends with a pair of steerable road wheels (not shown) through a pair of tie rods 8a and 8b so that when the steering wheel 1 is turned by an operator, the steerable road wheels (not shown) are appropriately steerable in accordance with the steering motion of the steering wheel 1 exerted by the operator. On the other hand, the steering rack 6 is operatively connected with a motor 13 through a second rack and pinion gear T.sub.2 including a second rack tooth portion 6b and a second pinion 17, a speed-reduction gear R, and a switching clutch 16 so that the driving force of the motor 13 is transmitted through the speed-reduction gear R, the switching clutch 16 and the second rack and pinion gear T.sub.2 to the rack 6 so as to assist the steering operation of the steering wheel 1 exerted by the operator. The motor 13 is electrically connected with a battery 11 through a control unit 9 and a key or ignition switch 12 so that it is energized by the battery 11 under the control of the control unit 9. The control unit 9 is input with control signals from a steering-torque sensor 3 and a vehicle-speed sensor 10 so as to appropriately control the operations of the motor 13 and the switching clutch 17 on the basis of the steering torque and the vehicle speed measured.
However, the power steering system as constructed above has the following problems. Specifically, as illustrated in FIG. 2, during transmission of the power-assisting force from the motor 13 to the steering rack 6, there will be a delay in operation caused by various elements such as the motor 13, the speed-reduction gear R including the worm 14 and the worm wheel 15, and the like, so that some time lag will develop until power assist to the steering motion induced by the operator is actually effected. Further, as shown in FIG. 3, the current supplied to the motor 13 generally fluctuates at a relatively long period t.sub.D1. Accordingly, due to such a time lag in operation in addition to the fluctuation of the relatively large period t.sub.D1 in the current supplied to the motor 13, the steering torque of the steering system power assisted by the motor 13 also fluctuates with a relatively great amplitude T.sub.S1, thereby causing the steering system to vibrate. As a result, smooth and stable steering operation is impaired.
Further, when the travelling speed of the vehicle increases above or decreases below a prescribed level, the control unit 9 operates to deenergize or energize the switching clutch 17 to interrupt or establish the transmission of the assisting force from the motor 13 to the rack 6 so as to switch the steering system into a manual steering mode without any power assist or into a power steering mode with power assist. Accordingly, whenever the vehicle speed changes across a prescribed speed level, the operating mode of the steering system abruptly changes from power steering into manual steering or vice versa, thus causing the operator to feel a certain unsmoothness, unsureness, or instability in the steering. In this case, even if such a change in steering mode is effected at the time when the steering torque is very small in order to avoid the above situation, there might be great shock developed due to the fluctuation in the steering torque and the time lag in operation when the steering mode is actually changed. This also impairs smooth steering operation.